1) Field of the Invention
The present invention is related to the use of diffusers to convert high-velocity fluids to high-pressure fluids, and more particularly to diffusers including a plurality of blades to direct and convert fluid flow.
2) Description of Related Art
Typically, a pump or compressor accelerates the velocity of fluid flow and then uses a diffuser to convert the increased velocity of the flow to an increase in pressure of the flow. For instance, a centrifugal compressor includes a rotating impeller having a plurality of helical blades that redirect and accelerate fluid flow from an axial direction to a radial direction. Around the periphery of the rotating impeller, and in fluid communication with the chamber in which the impeller blades are disposed due to its close proximity to the impeller blades, is a diffuser. The diffuser includes a plurality of static, radially extending blades bracketed by an upper and lower shroud so as to define a plurality of channels. These channels have an inlet end defined at the periphery of the impeller and extend to an outlet end at the outer periphery of the diffuser. Generally, the cross-sectional area of each channel increases as it extends from the inlet to the outlet, and as the fluid flows therethrough its velocity drops and its pressure increases. The relatively high pressure fluid is captured by a volute surrounding the diffuser. The volute is a scroll-shaped casing with a roughly circular cross-section having an area that increases as a function of wrap angle defined by the tangential direction of fluid flow emerging from the diffuser.
The operating range of most pumps and compressors is limited due to the instability resulting from stalling of the pumped fluid in the diffuser. Generally, stall is thought to be a result of the fluid flow separating from the suction side of the diffuser blades. In centrifugal pumps, separation of flow from the blades is more likely to occur when the angle of the leading edge of the diffuser blade differs greatly from the angle of flow of the fluid. Because variation in the velocity of the fluid exiting the impeller causes the angle of fluid flow to vary, the incidence of stall limits the range of speeds at which the impeller may operate. The range of impeller speeds over which stall does not occur is typically referred to as the xe2x80x9coperating rangexe2x80x9d of the pump or compressor.
In an effort to increase the operating range of pumps or compressors, multiple blades of varying angles can be used. U.S. Pat. No. 4,877,370 to Nakagawa et al. (xe2x80x9cNakagawaxe2x80x9d) discloses a diffuser for a centrifugal compressor that includes three differently sized and angled blades. As shown in FIG. 2 of Nakagawa, the blades consist of a plurality of main blades 7, inner sub-blades 8 and intermediate blades 9 at the outer periphery of the diffuser. Each intermediate blade is positioned so as to restrict the flow near the rear end of its adjacent main blade so as to prevent separation of the fluid from the main blade. Similarly, the sub-blades reduce the incidence of stall by being rotatably adjustable to more closely match the angle of the fluid flow at various operating speeds, as shown in FIGS. 8 and 9 of Nakagawa. Notably, U.S. Pat. No. 4,877,373 to Bandukwalla; U.S. Pat. No. 4,932,835 to Sorokes; U.S. Pat. No. 4,969,798 to Sakai et al.; U.S. Pat. No. 5,316,441 to Osborne; U.S. Pat. No. 5,320,489 to McKenna; and U.S. Pat. No. 5,529,457 to Terasaki et al. also disclose diffusers having multiple blade types for reducing the incidence of stall. Despite improvements in operating range, such diffusers are expensive to manufacture and the diffusers with moving blades are generally less robust than those with stationary blades.
Therefore, it would be advantageous to have a diffuser for use with a pump or compressor that efficiently converts high velocity fluids to high pressure fluids through a large operating range. Further, it would be advantageous to have a diffuser that is relatively inexpensive to manufacture and does not have a large number of moving parts so as to reduce maintenance requirements.
The present invention addresses the above needs and achieves other advantages by providing a diffuser for converting high velocity fluid into high pressure fluid. The diffuser includes a pair of spaced opposing walls between which extend a plurality of blades. Each of the blades has a pressure side and a suction side, wherein the pressure side of one of the blades is adjacent the suction side of another one of the blades. Thus, each pair of adjacent blades and spaced walls define a channel that extends from an inlet end to an outlet end. The cross-sectional area of the channel generally increases as it extends from the inlet end to the outlet end. As a result, high velocity fluid entering the inlet end becomes a high pressure fluid as it flows to the outlet end. Advantageously, the suction side of the blade has a height greater than the pressure side whereby the fluid is less likely to stall due to separation from the suction side. In addition, each blade preferably has a leading edge positioned at an angle of 10xc2x0 or less to further minimize the incidence of stall and increase the operating range of the diffuser.
In one embodiment, a diffuser is provided for converting a high velocity fluid into a high pressure fluid. The diffuser includes a pair of spaced, opposing walls, a pressure surface and a suction surface. The pressure surface extends between the pair of spaced, opposing walls and has a first height. The suction surface also extends between the pair of spaced, opposing walls, but has a second height that is greater than the first height. The walls and surfaces define a channel having a first end, a second end and a generally increasing cross-sectional area as it extends from the first end to the second end. High velocity fluid entering into the first end is converted into high pressure fluid as it flows along the channel to its second end by virtue of the generally increasing cross-sectional area. Advantageously, the height difference between the suction and pressure surfaces decreases the incidence of stall.
In another embodiment, a diffuser is provided comprising a pair of spaced, opposing walls and a plurality of diffuser blades. Each of the diffuser blades defines a pressure surface and a suction surface. The pressure surface has a first height extending between the pair of spaced, opposing walls. The suction surface has a second height extending between the pair of spaced, opposing walls wherein the second height is greater than the first height. The diffuser blades are spaced apart from each other so that the pressure surface of each blade is opposite the suction surface of an adjacent one of the blades. In this manner, the walls and surfaces define a channel having a first end, a second end and a generally increasing cross-sectional area as it extends from the first end to the second end. High velocity fluid entering into the first end is converted to high pressure fluid by virtue of the generally increasing cross-sectional area.
In yet another embodiment, an annual diffuser has a pair of spaced, opposing walls and a plurality of diffuser blades arranged in a generally annular, spaced relationship. Further, the blades are positioned so that a pressure surface of one blade is opposite a suction surface of another blade. The pressure surface has a first height extending between the pair of opposing walls while the suction surface has a second, greater height extending between the pair of opposing walls. A radially extending channel is defined by the walls and surfaces. The radially extending channel has a first end, a second end and a generally increasing cross-sectional area as it extends radially from the first to second ends.
In still another embodiment, a centrifugal compressor is provided for increasing the pressure of a fluid. The centrifugal compressor includes an impeller and an annular diffuser. The impeller has a plurality of rotatable blades defining a peripheral edge during rotation. Rotation of the blades accelerates the fluid in a radial direction so that fluid exits at the perhiperal edge. The annular diffuser includes a pair of spaced, opposing walls and a plurality of diffuser blades. The blades are arranged in a generally annular, spaced relationship so that a pressure surface of one blade is opposite a suction surface of another blade. The pressure surface extends a first height between the pair of opposing walls and the suction surface extends a second, greater height, between the pair of opposing walls. A channel is defined by the opposing surfaces and walls, wherein a first end of the channel is positioned adjacent the peripheral edge of the impeller so as to receive the fluid as it exits the impeller. The channel extends radially with a generally increasing cross-sectional area to a second end so that the fluid pressure increases as it flows from the first end to the second end.
In another embodiment, the blade height H(R) varies between the pressure surface and the suction surface as a function of the radius of curvature (R) wherein Rp  less than R less than Rs. Preferably, the variation ratio of the blade height is defined as H(R)/H=0.3535[(Rp+Rs)/R]3/2.
In yet another aspect, the leading edge angle of each of the blades is preferably less than approximately 10xc2x0 with respect to the tangential direction of the impeller blades at the adjacent peripheral edge.
The present invention has several advantages. The height variation between the pressure and suction surfaces in each of the channels reduces the likelihood of fluid flow separation at the suction side. Reducing the likelihood of separation reduces the critical incidence angle at which stall occurs, allowing for a larger operating range of the pump or compressor. Further reduction of the incidence of stall is accomplished by coupling the blade height variation with a leading edge blade angle less than about 10xc2x0. Further advantageously, the diffuser blades are stationary and at a single angle which is more cost-effective than diffuser designs having multiple blades at different angles and/or moveable blades.